Unlike SLi, you canâ€™t build a dual GPU CrossFire system out of any two cards. Instead you have to purchase what ATI calls a Master Card and marry this up to a normal ATI graphics board. At present, the master cards come in two flavours, Radeon X850 XT and Radeon X800 XL. Although the slave card doesnâ€™t have to be an exact match to the master card, I suspect that the system will run at the speed of the slower card. Although ATI has hinted at the fact that cards can run asynchronously, I didnâ€™t have time to put this to the test.
In order to put CrossFire through its paces I needed some hardware, and Evesham was kind enough to supply me with some. Evesham built a complete PC based on the CrossFire platform â€“ this consisted of an ATI reference motherboard for Socket 939 AMD Athlon 64, an FX-57 CPU and two Radeon X850 XT cards, one master and one slave.
When the Evesham machine arrived it fired up first time and proved to be rock solid throughout a complete run of benchmarks. However, when I came to actually play some games, things werenâ€™t quite so rosy. There seemed to be considerable tearing when panning around a scene, while turning on FSAA resulted in a horrible motion blur effect that produced a near instantaneous headache.
After much work and investigation by myself and Evesham we were unable to eradicate the tearing problem, although the motion blurring seemed to stop after I removed the graphics cards and re-installed them. Eventually we were supplied with a new driver revision by ATI which smoothed out the tearing and allowed me to get a better idea of what CrossFire is capable of - of course I re-ran all the benchmarks using the new driver.
Looking at the benchmark results first, thereâ€™s no doubt that CrossFire works, and two X850 XT cards definitely produce enough grunt for pretty much any game youâ€™re likely to throw at them. Running our custom Far Cry demo turned in a very impressive 85fps at a resolution of 1,600 x 1,200 with 4x FSAA and 8x AF, while in Doom 3, a game thatâ€™s traditionally nVidia friendly, the CrossFire machine managed 70fps at the same resolution and settings. Obviously I expected Half-Life 2 performance to be good and with a score of 81fps at 1,600 x 1,200 with 4x FSAA and 8x AF, I wasnâ€™t disappointed.